Such a circuit arrangement and such a method are known from EP 1 501 338 A2, in relation to which more details are given further below.
In order to operate a high-pressure discharge lamp, generally a sinusoidal AC operating voltage is required, whose frequency is swept in saw-tooth fashion in the range between 45 kHz and 55 kHz, usually with a 100 Hz clock, depending on the geometry of the high-pressure discharge lamp. The sweep operation generally prevents the permanent excitation of acoustic resonances and in addition contributes to the stabilization of the plasma arc (arc straightening).
In the case of high-efficiency metal halide lamps, the AC operating voltage should at the same time be amplitude-modulated in addition to the sweep operation in order to improve mixing of the fill, wherein the modulation should likewise be capable of being set corresponding to the geometry of the high-pressure discharge lamp, in particular of the lamp burner, both in terms of frequency, typically from 23 kHz to 30 kHz, and in terms of modulation depth, typically from 10% to 40%. The amplitude modulation in this case is used for targeted excitation of a special longitudinal acoustic resonance in the plasma arc which, with its property as the longitudinal mode, leaves the burning response of the plasma arc with respect to its stability unimpaired, but in addition brings about increased mixing of the gas components in the combustion chamber. This is known appropriately as color mixing. The amplitude modulation firstly results, in particular in the case of vertical operation, in a more homogeneous luminance along the plasma arc and secondly also in a considerable increase in luminous efficiency.
When using an inverter in a half-bridge arrangement for coupling the high-pressure discharge lamp to an electronic ballast, it is generally difficult to apply the amplitude modulation at this point. The amplitude modulation was therefore applied to the supply voltage of the half-bridge via a separate preliminary stage in the prior art, cf. in this regard DE 10 2005 028 4127.5. In terms of circuitry complexity, this requires at least one inductor and one or two electronic switches.
When using an inverter in a full-bridge arrangement for coupling the lamp to the electronic ballast, the amplitude modulation can generally be produced by phase modulation when driving the opposite corresponding electronic switches, as is described in EP 1 501 338, for example. In addition to the complexity involved in terms of two additional electronic switches for implementing an inverter in a full-bridge arrangement, this implementation has the disadvantage that the load circuit needs to be tuned to a sufficient depth for so-called zero-voltage switching to be capable of being maintained at relatively high inactive dead times in order to protect the field effect transistors, which are usually used as electronic switches. In addition, when using an inverter in a full-bridge arrangement, the lamp needs to be separated from the electronic ballast via a transformer owing to the steep edges at both outputs for reasons of EMC in order that only the harmonic differential signal now passes to the outside on the two lamp lines.